The invention disclosed herein relates to nozzles for vessels and piping that are installed either initially or as replacements without any welding, and to the installation of such nozzles without welding the nozzle to the vessel. (A "nozzle" may also be, or include as part thereof, a sleeve and/or piping. A "vessel" may also be large bore piping.) The invention more particularly relates to nozzles and procedures which replace nozzles that are attached to the vessel on the inside diameter of the vessel with a J groove structural weld, and has particular application to nozzles and procedures which replace or initially install nozzles in pressure vessels and large bore piping of pressurized water reactor nuclear power facilities which have failed due to phenomena known as Primary Water Stress Corrosion Cracking, PWSCC. The invention also particularly relates to nozzles and procedures for installing nozzles without welding as replacements or initial installations in large bore piping.
A typical nuclear power generating facility includes in part a reactor vessel, steam generator, pressurizer vessel, and a reactor coolant piping system, all of which operate under high pressure. Nozzles are attached to the vessels and/or piping for a number of purposes, e.g., for connecting piping and instrumentation, vents, and to secure control element drive mechanisms and heater elements. A typical pressurizer vessel 20 is shown in FIG. 1 with nozzles 22 for vents, nozzles 24 for liquid level, nozzles 25 for pressure sensing, a nozzle 26 for temperature measuring, and a number of nozzles 27 for heating elements. All of those nozzles were heretofore welded to the pressurizer vessel at the time of original manufacture.
As shown in FIG. 2, cladding 29 is welded to the interior of the pressurizer vessel which is made of carbon steel. The temperature nozzle 26 shown in cross section in FIG. 2, which is exemplary of the welded nozzles 22-27, passes through a hole or bore 30 in the pressurizer vessel 20 and is structurally welded at its interior end 32 to the vessel 20 with a J-weld 34 along the interior opening to the bore 30. The diameter of nozzle 32 is slightly less than the diameter of bore 30, so that there is a small annular space 36 between the nozzle exterior and the wall of bore 30. The J-weld 34 also functions as a seal weld to seal the annular space 36. A reactor vessel (not shown) similarly has nozzles represented by nozzle 26 in FIG. 2 welded thereto. The piping of the reactor coolant system (not shown) also includes similar nozzles welded thereto. Further details of pressurizer vessels, reactor vessels and coolant system piping, in particular, and nuclear power facilities, in general, are known to those of skill in the art.
As mentioned, the invention has particular application to the prevention of nozzle failures in nuclear power facilities due to PWSCC phenomena, which occurs on components having a susceptible material, high tensile stresses, and which are in a corrosive environment, conditions which primarily exist on nozzle penetrations in the pressurizer vessel, reactor coolant piping, and the reactor vessel. Such failures are manifested by cracking, which the applicant recognized resulted from high tensile stresses introduced by welds which structurally attach and/or seal the nozzle to the vessel and the corrosive effect of the coolant within the vessel. Such cracking occurs at the grain boundaries on the inside diameter of the nozzle material (Alloy 600) at or near the heat affected zone of the weld and propagates radially outward through the thickness of the nozzle which eventually leads to small leakage of the reactor coolant supply.
As indicated, nozzles of these types have failed over time and have had to be replaced, either because of a failure in the nozzle or the weld attaching and/or sealing the nozzle to the vessel. A typical replacement procedure in a nuclear power plant environment requires shutting down the nuclear power plant, removing the nozzle, which typically requires machining operations, and welding a replacement nozzle to the vessel or piping. The welded replacement nozzles currently in use closely duplicate the original welded nozzle they replace, except that they may be made of a different alloy, e.g., Alloy 690 (less susceptible to PWSCC) instead of Alloy 600, and may also be represented by the nozzle shown in FIG. 2. Other weld repair methods involve installing a thick weld pad on the outside of the vessel and structurally welding the nozzle to the pad, and seal welding the interior end of the nozzle to the vessel.
Replacements employing the above-described procedures in a nuclear power plant currently require a minimum of approximately fourteen days for some types of nozzles and are extremely expensive. Including the lost revenue resulting from plant shut-down, which may be as high as $750,000 per day, the total cost of each repair is millions of dollars.
The above-described nozzle replacement procedures and any other replacement procedure that requires welding the replacement nozzle to the vessel not only is time consuming and therefore expensive, but also exposes repair personnel to radiation hazards, particularly where the nozzle replacement method involves personnel entering inside the vessel to perform the replacement. Also, both the original welded nozzle and the welded replacement nozzle and method subject the nozzle to high residual stresses imposed by weld shrinkage. These high residual stresses increase the susceptibility to PWSCC. Thus, the welded replacement nozzle offers no improvement over the original nozzle in terms of expected life and reduction of failures, other than any improvement that may result from use of a superior nozzle material. Although, Alloy 690 material is less susceptible to PWSCC than Alloy 600, it is not known at this time whether the change in nozzle material alone will eliminate the possibility of nozzle failures. Furthermore, one utility that has replaced nozzles using the original design criteria and Alloy 690 material experienced failures in the weld material itself. Based on this information, an improved nozzle replacement method is needed.
U.S. Pat. Nos. 5,149,490 and 5,202,082 (both of Brown et al.) describe methods and apparatus for replacing a nozzle for a pressurizer vessel in which the replacement nozzle is threaded to the bore. Although the replacement nozzle of the '490 patent is mechanically attached to the pressurizer vessel, according to the '490 patent, welding is still required to provide the seal between the nozzle and the pressurizer vessel. Therefore, the residual stresses discussed above are imposed on the nozzle by the weld whether it be a structural weld or a seal weld.
In the replacement procedure and nozzle described in the '082 patent, the original welded nozzle is not fully removed, and a mechanical seal is made between the remaining cracked nozzle portion and the end of the replacement nozzle. Leaving part of the existing nozzle at the interior welded end of the nozzle may lead to a future failure because the existing failed portion of Alloy 600 nozzle which was not removed from the vessel has cracks near the existing J weld that may propagate out to the base material of the vessel and cause further cracking in the failed portion of the nozzle. Further cracking in the remaining portion of the failed nozzle would not likely result in reactor coolant leakage, and therefore might be justified for the life of the plant; however, a better design practice would be to remove the cracked nozzle to eliminate further degradation of the vessel. The procedure described in the '882 patent thus has the drawback that a portion of the failed nozzle remains structurally welded to the vessel and therefore continues to subject the vessel to the same stresses as the original nozzle. In any event, the remaining nozzle portion and the vessel portion surrounding the bore opening are subject to further degradation.
As far as the applicant is aware, the nozzles and replacement procedures disclosed in the '490 and '882 patents have not been used in a nuclear power facility.
The following U.S. patents disclose other procedures for replacing or repairing nozzles, sleeves or tubes which include welding: 4,255,840 (Loch et al.); 4,440,339 (Tamai et al.); 4,615,477 (Spada et al.); 5,091,140 (Dixon et al.); 5,094,801 (Dixon et al.); 5,196,160 (Porowski); 5,209,895 (Wivagg); 5,271,048 (Behake et al.); and 5,274,683 (Broda et al.). U.S. Pat. No. 4,826,217 (Guerrero) discloses a mechanical tube clamp for boiling water reactors. U.S. Pat. No. 5,278,878 (Porowski) discloses a method for reducing tensile stresses in the welded nozzles.
Also a method previously used in steam generator tube repairs has been proposed with certain modifications to the Nuclear Regulatory Committee for repairing a leaking nozzle. According to the proposal, a sleeve is rolled into an existing nozzle and deformed against the ID of the existing nozzle such that a seal is created between the nozzle and vessel. A similar design was also proposed for a plug. However, the Nuclear Regulatory Committee declined the proposals because that rolling technique causes high tensile stresses at the rolled transition region which promotes PWSCC, and because that repair method was only leak limiting which could allow the boric acid in the reactor coolant to erode a portion of the carbon steel vessel.
Nozzles are currently being replaced in pressurized water reactor (PWR) nuclear power facilities both because they have failed and as a preventive measure where a statistical analysis has indicated a high probability of a future failure. Nozzle failures and such statistically indicated failures have been occurring frequently enough to be a major concern for nuclear power plant operators (and owners) for a number of reasons including the high cost of repairs and the millions of dollars in lost revenue due to plant shut down. Therefore, there is a need for a replacement nozzle and a method for replacing nozzles that have failed or may fail in the future, that (a) reduce the time and expense required to make the replacement and (b) do not require confined entry into a pressure vessel, which reduce radiation exposure to the personnel performing the replacement, and (c) reduce the susceptibility to PWSCC and do not result in further degradation of the vessel, and accordingly reduce the risk of future failures. A similar need also exists for a nozzle for initial installation applications and a method of initially installing such a nozzle in a vessel.
The invention disclosed herein addresses the above-described needs and avoids the problems discussed above, and provides nozzles and procedures for installing nozzles mechanically in pressure vessels in nuclear power facilities (and in other fields) that do not employ (a) a structural weld or a seal weld, and (b) any part of an existing nozzle which is being replaced.